Help with Alert.show()

Question

I need help with Alert.show(). My goal is to display a message box when a button is pressed.
My program header:
<?xml version="1.0" encoding="utf-8"?>
<s:View xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
actionBarVisible="true" fontWeight="normal" overlayControls="false"
tabBarVisible="true" textAlign="right" title="tipCalc">
//// start of script section:
<fx:Script>
        <![CDATA[
        import mx.controls.Alert;
        import mx.events.CloseEvent;
// show a message box when button pressed
protected function about_clickHandler(event:MouseEvent):void
            Alert.show( "This is a message!", "Dialog Title" );    }
This results in the errors below:
Access of undefined property Alert.
Definition mx.controls:Alert could not be found.  

Answer 1

Well, me and my three days of Flash Builder are lost in the woods. I have no idea how to do what you suggest. The code Google's showing me seems to call for scrapping my working functional program and starting from scratch. For a message box.
Can I add a message box to my existing code?
Here's the header...
<?xml version="1.0" encoding="utf-8"?>
<s:View xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
actionBarVisible="true" fontWeight="normal" overlayControls="false"
tabBarVisible="true" textAlign="right" title="tipCalc">
And here's theclickHandler that should call the popup.
    protected function about_clickHandler(event:MouseEvent):void
                //  Call a message box.